Since there isn’t a lot of hempcrete out there (yet), the end-of-life hasn’t been studied extensively (others please chime in if you are familiar with sceanrios). For the biogenic part, it will decay at some point, yet the binder will hold onto the carbon until calcined again. So depending on how the hempcrete is processed at the end of its life, I expect that the decay time will be longer than just green hemp due to the alkalinity of the binder inhibiting decay.
So, even though at some point some of the carbon that was stored in the biomass will continue in the carbon cycle, the net carbon storage benefit is realized because the rotation time for hemp is less than one year. Thus a carbon that is stored today is will be replaced in a year. This quick table from Guest et al. 2013 give a nice comparison of carbon storage (GWP_bio) as a function of rotation time and period stored in buildings with the assumption that at the end-of-life the biomass is used as bioenergy. A negative number means net carbon storage over the time horizon, so with a 1 year rotation time, and storage for 60 years, 1 kg of CO2 absorbed during plant growth results in a net storage GWP of 0.5 kg, whereas if it was stored for 100 years, it would be nearly 1kg of net storage. Note that this number can be refined for specific species (i.e., hemp) and the results change meaningfully if a different midpoint indicator is used (e.g., 500 year global warming potential). So for measuring carbon storage, there’s a strong argument for not using the metric of 100-year GWP (even though its currently the standard), and a longer time horizon should be considered.
While that’s a bit long-winded of an explanation, the key takeaways are:
- At the end-of-life, the binder will retain the carbon indefinitely, while the biomass will decay at some point.
- Storage from hempcrete is realized no matter what happens at end-of-life as long as it is stored for more than one year.
- The longer it is stored in the building, the more carbon storage is realized (as measured by GWP).
- A 100 year time horizon is not the best metric for quantifying carbon storage and either longer time horizons should be used, or dynamic LCA.
There’s a good bit more research that needs to be done in this area, specific to buildings, which I am working on, so stay tuned for more updates.
I’ll also point to this other thread where we had a brief discussion about accounting for biogenic carbon.
